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What are the advantages of a Trimmable Horizontal Stabilizer?
What are the advantages of a Trimmable Horizontal Stabilizer (THS)? For example, a THS is used on some Airbus and Embraer aircraft. What is the most important advantage, and why don't some other successful airliners have this type of horizontal stabilizer?

Image used under CC BY-SA 3.0 (Source). Depicting the right side THS on an ERJ-170

Note: A Trimmable Horizontal Stabilizer (THS) differs both from a Stabilator and a horizontal stabilizer with a trimmable Elevator

I can't think of any airliner that doesn't have a trimmable horizontal stabilizer. Most aircraft in general have it,  right down to small GA planes. The method for trimming may be different, but it's still there. Are you asking about this particular style of trim,  where the entire stabilizer is moved instead of just the control surface or a trim tab?

Yes. The entire horizontal stabilizer is moved. And with regards to what you said about the airliners that doesn't have a THS , the dc-7 was a very succesful airliner but with fixed stabilizers, the Lockheed L-1049 Super Constellation also had fixed stabilizers. So...

The trim tabs can be seen in this picture for the DC7... Another view

L 1049 trim tabs visible in this picture

@kepler22b -- it's a jet age thing ;)

@RonBeyer: kepler22b is right - the rim tabs are for zeroing stick forces, which is not needed in hydraulically operated elevators. Both the L-1049 and the DC-7 had a fixed stabilizer, and that is what the question is about.

@UnrecognizedFallingObject: The Messerschmidt Bf-109 used an adjustable horizontal stabilizer for pitch trim. The angle of the horizontal stabilizer could be adjusted in flight via a handwheel to the left of the seat. - Whether this predates the jet age depends on whether you hold that the jet age began when the Me-262 was deployed, or when Willie first thought of a jet fighter.

@A.I.Breveleri -- never knew that -- it sounds like ol' Willy was thinking ahead :)

The main advantage is smaller elevator deflection angles. This comes handy in two cases:

When high-lift devices are deployed, the center of pressure on the wing shifts backwards by up to a third of wing chord. Fowler flaps add wing area aft of the trailing edge, and slotted flaps are able to generate high suction peaks. The result is a massive change in trim, and the empennage now has to generate generous downforce. Changing the lift on the empennage by elevator deflection alone will exceed the maximum practicable deflection angle and leave no margin for control. By adjusting the stabilizer incidence, the elevator can be held near its neutral position and has reserves for control.
In transsonic flight the elevator might not always have a linear characteristic. The contour break due to an elevator deflection induces shocks which in turn lead to flow separation which reduces the control effectivity and can even reverse the control characteristic. Since the transition from subsonic to supersonic flight shifts the center of lift backwards, the empennage needs to add downforce when the aircraft accelerates in the transsonic speed range. An elevator deflection might not be able to produce the desired lift change, and only adjusting the stabilizer such that the elevator can be held neutral can restore trim and control effectiveness.

Older airliners from the propeller age had lower wing loadings and less powerful flaps. The center of lift on the wing changed less with flaps, so a fixed stabilizer was sufficient. But once the wing loading goes up to jet levels, and the wing is fitted with slotted fowler flaps, a moveable stabilizer is unavoidable.

Now remains the question why keep the horizontal stabilizer with separate elevator. Supersonic aircraft generally don't and move the whole surface, but all airliners keep it split.

@JanHudec the answer is "actuator dynamic range" (or bandwidth, if you prefer)

@JanHudec: Adding some camber by deflecting the elevator makes the stabilizer-elevator combination more effective. The whole tail surface can be made a bit smaller without compromising control power.

@JanHudec -- the TriStar used a full-blown stabilator -- its just that the THS is much more well-entrenched in the halls at A, B, that other B, and E ;)

Trimmable horizontal stabilizers (THS) are found in majority of airliners and large transport aircraft (like C-17 globemaster III, for example). They are usually part of the trimming system of the aircraft, unlike the elevators, which are controlled by conventional pilot inputs (like yoke etc.). The THS offers some important advantages like:

The required elevator deflection angles are smaller in case of trimmed aircraft and the system has full elevator deflection angles at extreme trim angles.
If the stabilizer is not trimmed, the (human or auto)pilot has to continuously adjust the controls to prevent the aircraft from pitching up or down more than required.
Aligning the elevator with the stabilizer reduces drag.
It allows for a wider range of c.g. movement compared to the elevator-trim tab system.

The decision to use a THS (or not) depends on the design. For example, the BAe 146 had a fixed tail plane in order to reduce complexity (achieved in part by the elimination of leading edge slats). The THS does add more complexity to the system and has been involved in a few accidents, like the Alaska Airlines Flight 261 and China Airlines Flight 140.
The stabilator or all moving tailplane is entirely different (with no elevators) and is mostly used in supersonic aircraft. It is used mainly to overcome the problem where elevator becomes unusable due to shockwaves produced by tailplane and the problem of Mach tuck. Combat aircraft to use them as they create a large pitching moment for lesser control effort. Also, they are used for roll control via differential movement.
In civil aircraft, Lockheed L-1011 Tristar used stabilators.

Image from tristar500.net
It is also used in some GA aircraft (like Piper Cherokee); the strong control response of the stabillator can result in overcorrection- this is overcome in part by using an anti-servo tab.

It is difficult to tell in that image, but it looks to me like the stabilator has more curvature in the lower (deflected) image than it does in the upper image. Is that true, or is it just an effect of the viewing angle?

@FreeMan, the difference in curvature is real and is due to the deflection of a mechanically-linked segment of the stabilator.

Yay me!  :)  Thanks, @Porcupine911

There are 3 reasons for the existence of a THS.

The large speed range of jet airplanes.
The large trim changes with changes in the wing configuration, slats flaps etc.
The large range of CENTER of GRAVITY that is possible.
to reduce the drag produced by the tailplane especially during cruise, thus improving range significantly.

This does not really seem to add anything to the other answers already posted. Please consider elaborating and adding relevant sources to your answer so that it can actually function an an independant answer to the original question.

Some EASA questions for ATPL mention the fact that stabilizer trim is less sensitive to flutter when compared to elevator trim

There are two basic advantages to a trimmable stabilizer:

greater trim range than other methods.
less drag than other methods.

These advantages are most significant in large airliners, supersonic planes and those rare tailless wings with washout which "put the tail at the ends of the wings". Other answers expand on some of these applications.
The downside is increased structural weight, as the structure supporting the whole stabilizer is very small yet must be strong and rigid in two axes while pivoting about the third. So airliners that can live without it are usually happy to do so.


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For a large commercial plane on landing, does the L/D ratio increase, decrease, or not change much?
Consider a large commercial aircraft coming in for landing. The exact part I'm interested in is the flare (where the aircraft noses up yet does not actually go higher above the ground), with flaps extended, but before any ground effect comes into play.
Compared to normal cruise, does the Lift-to-Drag ratio increase, decrease, or not change much at all?
Increasing angle-of-attack increases both the lift and the drag, but probably not by the same proportions. I'm not really sure which way the curve goes for a typical airfoil.
Also, there's a really harsh complication: the flare also results in the engines pointing upwards too. This means that some "lift" is generated directly by the engines' vertical component of thrust. It's not really lift; a more accurate way to say it is the wings don't need to produce as much lift as before, because the engines do a direct part in helping to counteract gravity.
So that might suggest the L/D in fact decreases, which seems so counter-intuitive.
Actual numbers for the L/D at both stages of flight would be the most rock-solid answer possible. But I wonder if the L/D's cited by the manufacturers are derived from a simple wind-tunnel test (at different AoA and flap configuration) or if they actually take into account the upwards-pointing engines and their typical thrust during flare (which would be best measured as a percentage of aircraft weight at that point, and the mess gets messy).

The pilots of "large commercial aircraft" often don't do much in the way of a flare. That is to say, there is often a minimal change in pitch. Even for smaller multi-engine aircraft like a Piper Aztec, we don't flare in the same way as with a Piper Cherokee. Reference this video of a 737 landing. Note the absence of net pitch change.

In the flare you are already in ground effect. Maybe we should compare the final approach with cruise.
The answer is simple: L/D is far worse in approach configuration. The wing buys the increased lifting capacity with a relatively higher drag increase. This is not all bad: If the L/D is too high, a precise landing becomes much harder. Also, some more drag allows you to run the engines above idle, so spooling up for a go-around takes less time. Generally, you want the L/D to be between 5 and 10 for landing. Too much, and the approach becomes too shallow for a precise selection of the touchdown point, and too little and you lose too much energy in the flare, so you decelerate too fast in that final phase where sink speed is reduced.
Compare that to the L/D of modern airliners in cruise, which is around 16 for designs like the Boeing 747 and above 20 for the Boeing 787 or the A350.
You can study this cheaply in gliders. Take one with powerful flaps, like the ASW 20 or the ASH 25. Try to land them with all flaps and spoilers fully extended: You will never get to a smooth touchdown, but drop on the wheel somewhere before the flare is complete. Then try to land them with flaps set for slow flight and no spoilers: You need a loooong runway to do so.
The lift contribution of the engines during the flare is very moderate. They do not run at full power, and even if they did, their combined thrust would be only maybe 30% of the aircraft weight. Since they don't point straight up, but only by about 10°, the vertical component would be only 5% even at full thrust. In reality, they run at a little above idle, and their lift contribution is only 2% at most.

Flap type effectiveness comparison (picture source). The L/D is only for maximum lift, not at 60% of it which would be more representative for an approach lift coefficient, but the trend to lower L/D with higher maximum lift coefficient should be obvious.

I could have sworn I've seen flares at some pretty "early" altitudes in final approach at my local international airport. I just googled what altitude does ground effect begin and couldn't find a number. Maybe it begins much higher than I thought?

@DrZ214: It is generally agreed that it starts to be felt at an altitude equal to the chord of the wing. The goal is to fly close to the ground and flare so the sink speed stops when the wheels touch the ground. Flare too early, and you are left high with too little speed. Bad idea!

@DrZ214 Ground effect varies by aircraft wingspan. The strength of ground effect is relative to the distance from the ground, and starts at one wingspan distance. See this Boldmethod article for more details with good illustrations and graphs.

@peter-kämpf I think that you mean about one wingspan.  If it were equal to wing chord, many aircraft would never encounter ground effect at all.

@PeterKämpf Regarding your comment, and apart from the errors already pointed out regarding ground effect, you are describing landing technique for light aircraft, not for large commercial aircraft.

I also want to point out that the table is missing one key column: AoA where L/D is maximized (but this needs a given airspeed too). Right now it's showing L/D at CL max (which oddly doesn't give an airspeed either). They're not bad info at all, but I was under the impression that increasing AoA of the wing increases both L and L/D up to a certain point. You mentioned 10 degree flare in your answer. Could that be it?

@JonathanWalters: Your source and Wikipedia seem to follow a meme that I have not encountered so far. I rely more on scientific literature, and while there are both a span-dominated and a chord-dominated ground effect, the consensus is that the ground effect really starts only below an altitude equivalent to the chord length. Also, I would be interested to learn how landing large aircraft differs from landing light ones.

@Adam: No. The span-dominated ground effect is small compared to the chord-dominated ground effect. Try to compare aircraft of similar chord but different wing span. What does really change with wing span is the effect of wind turbulence. Below one wingspan the aircraft starts to fly much smoother even in rough, gusty wind.

@peter That sounds like a good new question!

@peter For more scientific literature on ground effect see Rozhdestvensky's book if you would like.

@peter-kämpf you had said that at flare you are in ground effect.  Few aircraft wings are within 1 chord of the ground at flare, so if it has a range of one chord this has to be incorrect.  While ground effect falls off quickly with altitude, and it is not simply present or not, one wingspan is a useful approximation for the range of ground effect.

The L/D ratio is usually lesser i.e. worse in landing configuration comapred to cruise. Basically, the high lift devices in the wing trade off some effeciency (L/D) for extra lift. In a sense, this is what you want because you're more concerned with getting more lift at lesser speed at that point.
The following table from NACA Technical Note 1930 gives the change in the L/D ratio in different configurations compared to cruise.

Image from An Analysis of the effect of Lift-Drag ratio and stalling speed on stalling speed and landing flare characteristics by J.Calvin Lovell and Stanley Lipson
NASA used a CV 990 aircraft to carryout low L/D ratio apprach and landing studies, which indicate that under landing conditions, as the flaps are extended, the L/D ratio becomes worse. Data were obtained with 100-percent spoiler (speed-brake) deflection at
flap settings of 0$^{\circ}$, 10$^{\circ}$, and 27$^{\circ}$ with 100$^{\circ}$ spoiler (speed brake settings).

Image from NASA Technical Note D-6732 Low Lift-to-Drag ratio Approach and Landing studies using a CV-990 Airplane by Berwin M. Kock, Fitzhugh L. Fulto11, Jr., and Fred ]· Drinkwater III
Due to aerodynamic effects, they went on to use 36$^{\circ}$ flaps and 25 percent of spoilers instead. Data from NASA wind tunnel models also indicates that as the flaps are extended, the L/D ratio decreases.

Image from Influence of Low-speed Aerodynamic Performance on Airport Community Noise by Andrew March; Data from NASA Technical Paper 1580 Low-speed aerodynamic performance of a high-aspect-ratio supercritical-wing transport model equipped with full-span slat and part-span double-slotted flaps by Morgan, H. L., Jr. and Paulson, J. W., Jr
The first reference has data for other Boeng aircraft (727,747 etc) which also indicate that the L/D ratio suffers as the flaps are extended.
In a simple sense, we can think like this- the aircraft is flying close to the maximum L/D ratio (for propeller and large bypass commercial jets) during cruise. Any change from this configuration will most probably result in lesser L/D ratio. This is what happens during landing.
The engines contribute only a minimal amount to the 'lift' during landing. There are a few reasons for this. The thrust is usually quite a bit smaller than the lift. Added to this, the engines are usually not at their full power during landing of commercial aircraft. Plus, pitch angle is pretty small during landing in normal aircraft. Putting all these together, the contribution of thrust is usually negligible.

Your penultimate and ultimate paragraph make sense. I'm still trying to make sense of the charts because I don't know what the subscripts a, b, v, f, h, v are. But I just realized that large commercial aircraft should be low on fuel during landing, meaning much lower aircraft weight. No wonder engine thrust is so little at landing.

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From this cockpit picture I cannot identify this aircraft. Any ideas?
It seems a kind of hybrid for testing. The overhead panel looks similar to BAe 146, Any ideas?

That is the cockpit of the Shuttle Training Aircraft, a modified Grumman Gulfstream II. Here is higher resolution picture:

(Wikipedia)

The STA is a "highly modified Gulfstream-2 aircraft," Nickel said. The plane’s cockpit has been adapted to closely resemble a Shuttle's flight deck; the left-hand seat features the same hand controls that a Shuttle commander uses during a mission. Several passenger seats in the back of the aircraft have been replaced by a bank of Shuttle-simulating computers.

(NASA)
They could simulate the glide of the Space Shuttle by flying with the main gear lowered and reverse thrust in flight:

In order to match the descent rate and drag profile of the real Shuttle, at 37,000 feet (11,300 m), the main landing gear was lowered (the nose gear stayed retracted due to wind load constraints) and engine thrust was reversed. Flaps could be deflected upwards to decrease lift as well as downwards to increase lift.

(Texas Air and Space Museum)

They could simulate the glide of the Space Shuttle by flying with the main gear lowered and reverse thrust in flight I think this is by far the best description of the shuttle's flight dynamics I've ever heard.  I'm given to understand that there's nothing quite like falling out of the sky in an oil tanker with nothing but two penguin wings and luck to stop you from burying into the dirt...

It may have been better than being shot round the earth in a tin can on top of a ballistic missile.

Flaps could be deflected upwards to decrease lift as well as downwards to increase lift. Am I mistaken in thinking that flaps don't usually deflect upwards?

You are not mistaken. Note the phrase "highly modified."

@zymhan: Depends on context. As John mentioned, this was a special aircraft. Personally, as a glider pilot, "negative flaps" are an extremely common thing.

There's something fascinating about the white handwritten labelling above the displays.  All the costs, and some dude with a white-out marker and a steady hand is still a critical component.

@J... I've heard the landing described as trying to land a brick. With a glide angle around and upwards of 20 degrees, it doesn't really look and feel like gliding at all.

@Mast Yes, I think at 20 degrees it qualifies more as a fallslope than a glideslope

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Are there any programs to build a hydrogen powered airplane?
So called "renewable energy" is a buzzword these days. Auto manufacturers are starting to roll out hydrogen powered cars. Hydrogen may not currently be the most efficient or economic fuel for jet engines, but that doesn't mean there are companies working on this issue or even a new kind of engine.
Are there any such programs in existence today?

It should be noted that hydrogen isn't actually a "renewable energy" source at all. In fact, it's not usually even an energy source in the first place. It's usually an energy storage mechanism for energy produced from some other source (usually the electric grid, which means mostly coal.) Much like a battery, just a lot harder to contain but with less harmful waste products.

@reirab - By that logic, the only true energy source is the sun. With the exceptions of nuclear and geothermal (both caused by radioactive decay of heavy elements, and thus given to us by some other star) and maybe tidal harnesses (caused primarily by the moon's gravitational pull on the oceans, but that still requires liquid water and thus a heat source), all the "energy sources" we currently have, we have because that source captured and stored the sun's energy (the water cycle for hydroelectric, photosynthetic reactions for most fossil fuels).

@KeithS The difference is that all of those things are done by nature, whereas Hydrogen has to be created anthropogenically using energy from one of those other sources.

Aviation is very energy-hungry. While hydrogen is quite efficient in terms of energy per mass (141.8 MJ/kg), it is awful in terms of energy per volume. Aircraft today use their wings for fuel storage, which helps to reduce bending loads and uses a volume unsuitable to transport payload. This will not work for hydrogen.
The biggest problem is storage, and none of the solutions is really attractive:

Cryogenic: Storing the hydrogen at a temperature where it is liquid (below 20°K) greatly reduces volume demands, but now you need to add insulation, and even then the energy per volume is only a quarter compared to hydrocarbons. This is used in rockets, because they can rely on extensive ground support before launch and use up the stuff in a few minutes. Airliners, however, need to store their fuel over tens of hours, so their hydrogen tanks will be much heavier and bulkier than those of rockets.
Compressed: This avoids the energy input for liquefaction, but now you need a strong and heavy pressure vessel. Current gaseous hydrogen storage uses up to 700 bar pressure, but even then the volume demand is six times higher than that of kerosene for the same energy content.
Hydrides: Some metals like magnesium or sodium can bind hydrogen and release it when heated. However, even in the most efficient compounds like sodium tetrahydroaluminate, only 7.4% of mass is useable hydrogen, making this exquisitely unsuitable for mass-critical applications like aviation.

Currently a mixture of cryogenic and compressed storage looks the most attractive and has been tested widely.
The first (partially) hydrogen powered aircraft was the LZ-127 Zeppelin which used a mixture of gasses of equal mass per volume as air (Blaugas) to power its engines.
Tupolev designed and flew the Tu-155, the first hydrogen-powered heavier-than-air aircraft, in 1988. The results showed that the principle worked, but could not compete with kerosene.
Currently, several demonstrators are in operation, but none with the hope of widespread application - as it stands, binding hydrogen chemically to carbon will still give the best way of storage (liquid and unpressurized) for aviation.
Hydrogen becomes the best source of energy only when combustion speed is critical, such as in supersonic combustion ramjets (scramjets).

So, basically, it boils down to the fuel alone has a better energy/mass ratio, but the fuel + storage system for the fuel has a worse one (and/or requires more bulk which adds unacceptable amounts of form drag)? And this is all aside from added design complexity and the resulting increased chance of something going wrong, of course.

To add numbers, density of kerosene is ~800 kg/m³ while liquid hydrogen only has ~70 kg/m³ (compressed to 700 bar approaches that value), so 11.5 times higher volume is needed.

do keep in mind that compression and liquification are effectively one and the same, the insulation and cooling requirements are thus very similar, as are the requirements on the pressure vessel.

To add even more numbers: Compressed gaseous hydrogen has a density of only 40 kg/m³ at 700 bar. Compressing it from 350 to 700 bar raises density only by 67% since with increasing pressure it will behave less and less like an ideal gas.

Another data point on the curve: the Saturn V's third stage used (liquid) hydrogen as a fuel, but by the time it ignited, the craft was no longer an aircraft.  They'd hoped to use hydrogen for the second stage as well, but that would have been too tricky and expensive, even at that grand a scale.

Airbus have claimed they aim to produce hydrogen-fuelled passenger planes that could be in service by 2035. They have developed three ZEROe concept designs.

Unveiling its latest blueprints, Airbus said its turbofan design could
carry up to 200 passengers more than 2,000 miles, while a turboprop
concept would have a 50% lower capacity and range.
A third, "blended-wing body" aircraft was the most eye-catching of the
three designs.
All three planes would be powered by gas-turbine engines modified to
burn liquid hydrogen, and through hydrogen fuel cells to create
electrical power.
However, Airbus admitted that for the idea to work, airports would
have to invest large sums of money in refuelling infrastructure.
"The transition to hydrogen, as the primary power source for these
concept planes, will require decisive action from the entire aviation
ecosystem," said Mr Faury.
"Together with the support from government and industrial partners, we
can rise up to this challenge to scale up renewable energy and
hydrogen for the sustainable future of the aviation industry."
The new Airbus designs are the fruit of a joint research project that
Airbus launched with EasyJet last year to consider hybrid and electric
aircraft.
The airline's chief executive, Johan Lundgren, said: "EasyJet remains
absolutely committed to more sustainable flying and we know that
technology is where the answer lies for the industry."

BBC News: 'Airbus looks to the future with hydrogen planes'

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Why aren't PFDs made as robust as the Integrated Standby Instrument System (ISIS)?
The integrated standby instrument system (ISIS) is a standby instrument used in many large airliners that provides airspeed, altitude, and attitude information to the pilot in the case of electrical issues. The ISIS is an isolated system that uses its own pitot and electrical. If the ISIS can survive such dramatic failures, why don't we put that technology in the PFDs (Primary Flight Displays) so they to can maintain operation when electrical systems are down?

Its not that the Integrated Standby Instrument System (ISIS) is infallible, its that its failures are unrelated to the main system.  ISIS can and will fail from time to time.  But its failures should be unrelated to the main instruments, so that one of the systems is always working, and the chances of both failing at the same time is extremely remote.

@abelenky, I'd upvote that as an answer.

You may want to expand more acronyms, I came from HNQ having read PDF as Personal Flotation Device,

The value of the Integrated Standby Instrument System (ISIS) is not that it is infallible.  Its value comes from its failures being unrelated to the primary system.
ISIS can and will fail from time to time. Its pitot system can get blocked, and its electrical system might have faults.  But its failures should have no correlation to the main instruments.  Ideally, both Primary and ISIS will work at all times, but if one system fails, the other one should work as a fully independent, unconnected system. The chances of both failing at the same time is extremely remote.

Would it be a waste to give the PFD a backup battery for an extra layer of redundancy?

@Boeing787: Depending on the plane there is a very good chance that the PFD can already be powered by battery, by APU, or by the alternators on engine 1 or engine 2.  Another battery would add a small bit of redundancy, at the cost of added complexity, but leave 500 other failure pathways unchanged.

And since the black boxes (CVR and FDR) can survive a crash, why not make the entire plane out of the same stuff?
Airplanes, as with nearly any complex technological system, is a balance between cost and benefit. Cost, in an aviation context, includes both direct $ costs and also the indirect cost of additional weight. Batteries sufficient to power large systems (primary flight displays and the avionic systems that provide the data form them) for a long time are heavy. Wiring, control systems, etc. all add to the weight. The benefit, in the case of a higher level of power backup for the PFD and related systems, is minimal, as the failure rate is relatively low (hopefully, by design) and ISIS needs to be there anyway for other types of failures. Plus with modern airliners there is a lot more that needs power than just the cockpit displays, which can be provided, in many airliners, by a Ram Air Turbine. If the RAT deploys then the key avionics will have power even if the engines can't be restarted, and key flight control surfaces will have power too.

The "data" is recoverable from the "black boxes". The boxes themselves are seldom in a working condition after a crash. They are build solidly enough not to get broken into thousands of piece, but to stay together enough they can be located. They also have a certain amount of fire resistance.

@Jim I am well aware of all of that. And the internal components can handle a lot more Gs than a human in a steel box ever could, so even if weight wasn't an issue, a big steel box around each passenger would not protect them effectively from a high-speed crash.


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What are the reasons behind this pusher propeller configuration?
Can a pusher configuration sort of like this have its propeller close to the tail (if it were one of those split tails like on a A-10)?

Why did they put the propeller above the cabin and not in the tail?

A heavy, vibrating engine in the tail is a bad idea. Any frequency neighbourhood will lead to flutter. It is better to place it on a dedicated post. Also, a propeller in the tail stabilizes the aircraft, so it will need bigger control surfaces.

Related: https://aviation.stackexchange.com/questions/30146/is-there-really-anything-to-be-gained-by-placing-the-propeller-at-the-top-of-the

@PeterKämpf - "... a propeller in the tail stabilizes the aircraft ..." did you mean "destabilizes"?

@KevinFegan: No, it stabilizes the aircraft. For more, I recommend this NACA report titled "propellers in pitch and yaw" which should prove the point. Or look here.

Yeah good question. They need it to be out of the way of the spray, so high up, and they position it on top of something that looks like a vertical stabiliser, so why not use the vertical stabiliser, I presume is what you mean. Like this one, but then in a pusher comfiguration

My answer would be that that would position the centre of gravity too far back. The engine is heavy, it's best if it is mounted close to the centre of the plane. The combination of pusher prop and tail mounting would make for a configuration that wants to tip backwards. Placing the wings further back is awkward, it's a small plane and leaves little moment arm for the tail surfaces to work effectively.

Can you identify that aircraft please?  I'm fascinated by it.

@Criggie - Google reverse search says it's a A-90 Orlyonok.

Engine weight produces torque raising the nose, but placing the propeller high produces torque raising the tail. Therefore, combining high and back seems to make sense.

@Criggie They are fascinating planes aren't they, some of the ekranoplans are just pure science fiction.

@Pere yes get the total cog including engine somewhe around mid frame, and account for nose down moment with increased throttle. Good for anti-stall.

The heaviest single item in an aircraft is the engine. If you were to move the engine to the tail and make no other changes to this design the aircraft's centre of gravity would move so far aft that the aircraft would be unflyable.
For a stable configuration the centre of gravity needs to be close to the centre of lift. We could achieve that by moving the wing aft as well, so that the weight of the fuselage ahead of the centre of lift balances the weight of the engine behind.
Now, though, you have almost no pitch control from the elevators. We can counter that by moving them forward (where they're called a canard) and now you have this design:

Rutan Vari-eze
Source Wikipedia
However, your aircraft also has to be a boat. Placing a large weight at one end of a boat will make its handling problematic. Better to place the heavy weight in the centre of a boat, and we're back to the original design.
The engine is placed high to keep it clear of the water, and away from possible spray during landing and take-off.

Heavy weight in the back is preferable for a planing boat. Speedboats have no problem with rear mounted  engines.

Yes, but you're designing a part-time boat - the rest of the time it's an aircraft. And aircraft are awfully picky about the position of their center of gravity relative to the wing. Put the engine all the way in the back and you're going to have to move the wing all the way to the back as well.

+1 for pointing out its a boat, that enters and exits the water at speedboat velocities.

Also, if you had a puller in that configuration, the pylon would need to be taller, or the prop would hit the cabin.

At first sight, placing the propeller in the tail looks attractive, but there have been very few designs which put that into practice. The best known is probably the Trislander derivative of the venerable Britten-Norman Islander utility aircraft.

Britten-Norman Trislander (picture source)
Disadvantages of a tail-mounted propeller engine
To place a heavy mass, part of which is even rotating in a wide band of frequencies, on a long, flexible beam is generally not a good idea.

The engine mass will reduce the eigenfrequency of the vertical tail, inviting early flutter, and any imbalance in the propeller will provide the initial excitation which flutter needs to develop.
Placing heavy masses away from the center of gravity increases the moments of inertia and makes the handling of the aircraft needlessly sluggish.
Doing so in two axes invites a phenomenon known as inertia coupling.
Next, a propeller acts like a tail surface. Placing it far aft will stabilize the aircraft and make it harder to control; generally such a propeller location must be combined with larger tail surfaces.
Placing the propeller high above the center of gravity will also result in trim changes whenever power is changed. This is less than welcome if good handling characteristics are desired

Only seaplanes accept a high propeller location to make sure that spray will not hit the propeller. On the Dornier Do-26 the rear propellers could even be jacked up for take-off and landing.
Existing Designs
Recently, however, a few special airplanes have chosen a tail-mounted engine. The common factors were:

electric propulsion, so vibrations were low
low power loading, so the stabilisation and trim effects of the propeller were small
high aerodynamic and structural efficiency, so the extra post and nacelle for the engine were deemed undesirable.

You see two of them together below: Right is the Icaré II, the first practical solar aircraft, and left is the e-Genius, a battery-powered experimental aircraft.

Icaré 2 and e-Genius in formation flight (picture source)
Given the many disadvantages, designers generally try to avoid a rear propeller location. Pushers give better efficiency, so some tried to avoid a conventional layout. Below you see a compromise, where the propeller is mounted aft of the wing, but still close to the center of gravity. The position on the Lake Buccaneer is another such compromise and a good choice for an amphibian. The cruciform tail is even better than a twin tail because the propeller blast will hit all control surfaces, enabling much better low-speed handling on water.

HB-23 Hobbyliner in flight (picture source)
Note that I never mentioned the rear center of gravity location. This is really not much of a factor in this case.

Note that I never mentioned the rear center of gravity location. This is really not much of a factor in this case. Not with the planes with the long tail arm, or with the one with three engines in total. Mounting the engine on a vertical tail-like support causes flutter issues as well. The aircraft in question is short, stubby, light, and takes off from water. Good broad info though as usual. +1'd, working on the Sportsmanship medal which not many people have :)

@Koyovis: I upvoted many of your answers, but this time I simply was not happy with it. This is no competition, but a quest to answer other peoples' question as good as possible. Moving the wing a little back on the Buccaneer would clear the cabin area of the spar and reduce the tail lever arm only slightly, so it is not a bad solution. The real drawbacks are in handling and dynamic response.

Yes no worries, the info in your answers is always worthwhile. I take the question as why a pusher propellor is not mounted on top of the vertical tail - in that case the engine would stick out at the back, not in front of the tail. Plus I reckon that the engine might be a larger fraction of weight of a light plane.

Several reasons. The tail's height above water depends on the pitch up or down of the aircraft but you don't want the propeller too close to the water(!). You also don't want water spray hitting the propeller (the very high tip speed can quickly be eroded by hitting water spray) so keeping it closer to the nose and higher prevents that. Putting it near the wing means its weight and other forces are transferred more directly to the aircraft's strongest structure, the wing spars, keeping structural (and overall) weight down. The propeller is also aimed at the tail, giving it more control authority at low speeds (near stall, takeoff and landing, etc.).
Some electric aircraft designs do mount the propeller at the top of the vertical stabilizer as they have a lower weight motor instead of a heavy engine and a larger diameter propeller that needs to clear the fuselage and ground. In such aircraft, the heaviest item is not the engine (i.e. motor) but the battery pack which is usually mounted near the wing spar (often in the wings). It also allows the landing gear to be relatively short / light weight / low drag.

authors: Bob Jarvis - Слава Україні


text:
What is the ICAO equivalent of a "point out"?
In FAA air traffic control, when we have an aircraft that is transitioning through someone else's airspace that we want to talk to we do a "point out":

Point Out. A physical or automated action taken by a controller to transfer the radar identification of an aircraft to another controller if the aircraft will or may enter the airspace or protected airspace of another controller and radio communications will not be transferred. - source

Can anyone please tell me what the equivalent of a point out is in ICAO (with a reference to ICAO Doc 4444 if possible)?

Based on the comment by @mins, it seems this question is about transfer of identification, which is a way for one controlled to identify a track based on an identification already established by another controller.
In this case, the ICAO equivalent of a “point out” is explained in Doc 4444 section 8.6.3.2(e):

8.6.3.2 Transfer of identification shall be effected by one of the following methods:
[...]
e) direct designation (pointing with the finger) of the position indication, if the two situation displays are adjacent, or if a common “conference” type of situation display is used;
Note.— Attention must be given to any errors which might occur due to parallax effect

This is mostly a thing of the past, since most tracks today are identified based on SSR or ADS.


text:
What is Lower Traffic Area (LTA) airspace in France?
In France, AIP ENR-2.1 subsection 2.1.2 defines airspaces of type CTA (control area), UTA (upper traffic area) and LTA (lower traffic area).
I'm not familiar with LTA which appears not to exist outside of France and isn't part of AIXM neither. Can anybody fill me in on how LTA differs from CTA and how it could be coded in AIXM?

"LTA: FL 115 to 195, UTA: FL195 to 660" (Wikipedia). Created in 1992, a LTA is a class-D CTA (E over Alpes and Pyrénées to allow VFR gliders) between and excluding other CTA (airways and TMA), and up to 12 NM of the shoreline. I've highlighted a class-E LTA nearby Perpignan TMA and the Spanish border on this SIA map. Synoptics of the French airspace (source).

@mins Has answered the question in his comment above:

LTA: FL 115 to 195, UTA: FL195 to 660" (Wikipedia). Created in 1992, a LTA is a class-D CTA (E over Alpes and Pyrénées to allow VFR gliders) between and excluding other CTA (airways and TMA), and up to 12 NM of the shoreline. I've highlighted a class-E LTA nearby Perpignan TMA and the Spanish border on this SIA map. Synoptics of the French airspace (source).


text:
Is my understanding of magnetic variation and declination (for a VOR) correct?
AIXM (the XML-based exchange model for AIP by Eurocontrol and european AIS) defines the following properties for a VOR:

codeTypeNorthA code indicating the direction of the "zero bearing" provided by the station, values: "TRUE" (true north) or "MAG" (magnetic north).
valDeclinationThe angular difference between the direction of the "zero bearing" indicated by the station and the direction of the True North.
valMagVarThe local angular difference between the True North and the Magnetic North.
dateMagVarThe year the magnetic variation was measured.

Now say we have a VOR with the following values:
<codeTypeNorth>MAG</codeTypeNorth>
<valMagVar>0.27</valMagVar>
<dateMagVar>2018</dateMagVar>

Am I correct with the following statements?

valMagVar is a positive number, therefore magnetic north is east of true north.
codeTypeNorth is "MAG", therefore the VOR is aligned towards the magnetic north. Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

Please don't comment on the practical relevance of this question. I'm developing and documenting OFMX (a derivative of AIXM) we will use as our future well-defined exchange format for https://openflightmaps.org and I want to be sure to be 100% sure I explain the terms "magnetic variation" and "magnetic declination" accurately in the OFMX documentation.
Thanks a lot for your help!

Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

You are assuming that the VOR alignment will always be kept bang-up-to-date with the actual measured magnetic variation. Sadly, this is not the case! There is actually a considerable amount of work involved in re-aligning a VOR (it is not just a matter of tweaking a knob!) and re-alignment is only performed rather rarely.
Also note that the terms 'variation' and 'declination' are NOT synonymous in this context - 'variation' refers to the actual measured difference between true north and magnetic north; 'declination' refers to the difference between VOR zero-bearing and true north. These are NOT generally exactly the same.
For example: UK AIP ENR 4.1 (PDF)

In other words: The declination is the "correction" from the re-alignment of the VOR while variation is the actual difference between true and magnetic north. And theoretically, right after a re-alignment, variation and declination should be equal, but will diverge until the next re-alignment is made. Correct?

@svoop: correct

Great, thanks a bunch!

Link is now paywalled.


text:
Do small aircraft have sockets for passenger headsets?
I am currently training for my PPL. in the Cessna 182s we use I've only ever noticed two sets of ports for the headsets. If there were additional passengers in the back seats, how would they communicate?

The rear seats of a Cessna 172 has headset jacks, I imagine the 182 has the same.
You can see them in this picture:

Image source
You might also find them over the shoulder of rear passengers - as can be seen in this image (copyright attached)

That links the rear-seat passengers to the same intercom you're used to from a front seat passenger.

Pre 21st century the ports seemed to be an afterthought, I've seen them in various different places. In the roof, on the floor, & on the sides (as pictured).

@BDLPPL yep - Ive definately seen them on the ceiling.

I don't recall any of the C172s I rented having rear seat intercom. They were all 80s vintage C172Ps.

It depends on the airplane and what year was manufactured in. Generally though the answer is yes. Most modern general aviation airplanes include headset jacks at all passenger seats.   Diamond aircraft have the plugs for all headset jacks on the aft section of the center console.  Cessna has them on the wall liners of the airplane.   Many modern airplanes also will sell active noise canceling headsets included with the aircraft for all passenger locations.   They provide active noise canceling to quiet the flight, intercom communications for all people on board the airplane and offer entertainment options such as Bluetooth connection with mobile devices and onboard XM radio.


text:
Is my understanding of subsonic inlets correct?
For subsonic aircraft like commercial jets, the inlet has the job of making sure the correct airflow is coming into the engine. Correct airflow is important for keeping the engine's compressor stage working. Too little or too much airflow will cause the compressors to stall and therefore no combustion happens. The inlet must keep airflow constant throughout the different stages of flight, including take-off, landing and banking.
The way an inlet ensures correct airflow into the engine is by slowing down the airflow and increasing the pressure. The inlet has a large volume which will decrease the speed and therefore will decrease the pressure. An inlet will work differently during low speed flight and high speed flight:

Low speed flight – During low speed flight including take-off and landing, the aircraft isn’t going fast enough for sufficient airflow to enter the engine. All engines have an operating design where they need certain airflow to work. To get enough airflow into the engine at low speeds, the engine's fan will suck in air. Air will be sucked in from a large area in front of the inlet. The streamlines of air will converge together as they get closer to the inlet. However, there is a location on the inlet where air does not move. At low speed flight, this stagnation point is on the outside of the inlet. This forces oncoming air to enter the inlet and not go around the engines nacelle. This provides the engine with enough air velocity at low speeds to operate efficiently.
High speed flight – During high speed flight the aircraft is going fast enough for sufficient airflow to enter the engine. However, allowing too much airflow will cause the compressors to stall. The inlet isn’t relying on the fan to suck in air from all directions. Air directly enters the inlet because the aircraft's velocity is fast enough. So to stop too much airflow entering the engines, the stagnation point moves further inside the inlet. This means that the desired airflow enters the engine, and the unwanted airflow flows around the nacelle.

Is this understanding of subsonic inlets correct?

Your understanding is correct.
However, you attribute too much activity to what is essentially a passive device. The inlet only guides the air such that it flows uniformly, without separation and at the desired angle towards the fan and the compressor. The pressure field resulting from this flow is doing the acceleration rsp. deceleration, and the pressure itself is a result of the compressor swallowing just as much air as it needs.
Converging streamlines indicate a pressure drop and acceleration, and diverging streamlines a pressure increase and deceleration. In the static condition, the engine creates suction inside the intake which causes air from all around to flow towards the intake. At high speed it cannot swallow more than a fraction of the air flowing towards it and partially blocks the flow through the nacelle. As a consequence, air gets pushed to the sides and the flow into the intake is decelerated.
It is really the engine, not the intake, that controls how much air is ingested. The intake only makes sure that this happens in an orderly fashion without separation and vorticity. Different flight speeds only mean that the pressure ahead of the compressor changes, while the speed of the air entering it is quite constant over flight speed.


text:
Why don’t double-bubble fuselages cause severe fatigue problems?
In response to my previous two questions about fatigue issues, or the lack thereof, generated by the cusps in double-bubble-fuselaged aircraft (Stratocruiser, DC-9-80), I received a number of answers about how the presence of a load-bearing floor attached to the fuselage cusps, in the style of the film between two abutting soap bubbles negates any susceptibility to fatigue that the cusps might otherwise generate.
For instance, this answer by @MaxPower to the Stratocruiser question:

It is not the stress concentrating type of sharp corner. That is the natural shape that allows the parts to be in a nice smooth tension at lowest energy. If you made an elastic rubber balloon with a divider membrane creating two chambers and inflated it you would get a similar load distributing shape.

And these two from the DC-9-80 question, one of them by @JohnK:

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.
If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

and the other by @Nyos:

The answer is soap bubbles:
[image]
They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture) This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

And I sorta get what they’re getting at, but aircraft are not, generally-speaking, made of soap films, but, rather, of (mainly) aluminium alloys (in the case of most Boeing aircraft) or fiber-reinforced plastics (in the case of Airbus and newer Boeing aircraft), both of which, unlike soap films, are vulnerable to fatigue cracking.  I understand that the load-bearing floor stretched between the cusps is supposed to support them and make the cusped area no more vulnerable to fatigue than any other part of the fuselage, but it seems to me like that would only really work if the floor has exactly the same mechanical properties (elasticity, yield strength, etc.) as the outer pressure hull, which is somewhat unlikely.1
For instance, if the floor is stretchier than the pressure hull, pressurising the aircraft should cause it to balloon outwards around the tropic of Capricorn, causing considerable bending stresses in the cusped areas (although, admittedly, less than would be generated in the absence of a floor):

(The green arrows indicate what expands in what directions.  The red arrows indicate the direction of the resultant bending stresses on the cusp areas.)
Or, if the pressure hull is stretchier than the floor, the upper and lower lobes of the hull should balloon outwards, again causing considerable bending stresses in the cusps (although in the opposite direction this time):

(The green and red arrows serve exactly the same purpose as with the previous image.)
How and why isn’t this a problem in practice?

1: For instance, the pressure hull has to, as the name implies, resist radial and hoop stresses generated by cabin pressurisation, while the cabin floor only has to resist longitudinal stresses from same (which is why aircraft floors need blowout vent panels to keep a rapid or explosive decompression of the cargo hold from causing the floor to collapse and sever floor-routed flight-control cables); as materials react differently to being stressed in different ways, one of the two is likely to need a beefier structure than the other, making it more rigid and less stretchy.  Also, the corrosion environments experienced by the floor and the pressure hull are considerably different for most aircraft, likely causing one to corrode (and become weaker and less rigid) faster than the other.

Anonymous downvoter, please state your concern.

@Koyovis: Good to know I'm not the only person who sees that that's a problem!

Because the effects you describe are too small to place enough bending stress on critical locations to be a significant problem, and to the extent that is a problem, you simply add extra meat as required.  If the fuselage grows say 50 thousandths in diameter at max differential and the floor beam stretches by say 20 thousandths, the change in the geometry of the Y intersection is fairly insignificant and well within the ability of the material to flex without accelerated fatigue. And in any case, as a joint, there will be more meat in the structure as a matter of course, done in a way the feathers the stress away from the center of the Y to the extent that there is any local bending because the bubble grows more than the joining tension beam (in any case, one leg of the Y, the floor beam, is getting bent downward by the load of the pax, so you are still stuck with accounting for bending in the joint just from that).
You can make pretty much any structure fatigue proof if you want by adding meat. You will see transport airplanes with flat pressure bulkheads.  Lots of bending stress going on, and the flat skins of the bulkhead want to oil can (bulge).  You just have to make the beams heavy enough to take the loads without too much deformation, and feather structure away from peak stress points as much as possible to save weight (including making the skins thicker at beams and stiffeners and thinner in the middle, usually done by chemical-milling).
It would be lighter to make a pure tension curved bulkhead, but sometimes you are stuck with that configuration (and in fact almost all airliners have at least one flat pressure bulkhead, at the very front). In any case, even the skins in a circular pressure hull also bulge a tiny amount because the skin is anchored to frames and stringers that stretch less, and they are also may be chem milled to make them thinner in the middle of panel sections to save weight and thicker at the rivet lines because there is a tiny amount of oil-can bending).
The art is in making everything just strong enough to meet your requirements (crack free over X thousand cycles), because every ounce over that is ballast.  When you do your long term fatigue test of the fuselage barrel, you find out of the calculations you made to size all the material was too much or not enough.  If not enough, you end up with mod campaigns.

Aluminium and its alloys (which most aircraft are made of) don't have any ability to flex without accelerated fatigue, unlike (say) structural steels, which have a so-called fatigue limit.

Ok maybe not "fatigue proof", but you make the material thick enough the accelerated fatigue is pushed so far out it might as well be.  The DC-3 has an indefinite fatigue life because the structure in key areas is heavy enough to be close to the near vertical part of the fatigue curve for aluminum.

...and because it isn't a pressurised aircraft, so its fuselage skin doesn't have to deal with the stresses of being inflated and deflated with every flight.

Airliners have fatigue issues beyond the pressure hull. Gear attachments, beams, etc.   The reason is the DC-3 was designed when aluminum fatigue science was in its infancy so very large fudge factors were included.  Modern airplanes are designed to a specific cycle limit, with 3x safety factor applied to the actual fatigue test results if cracks appear, to allow for scatter.

But the wings flex a LOT more, pressurized cabin or not.

The flex is distributed down the span to avoid concentrating stress, and where you do have a concentration such as at an attachment, you just add extra meat, feathered into the adjacent structure to distribute the concentration gradually.

@JohnK: Yes, other parts of the airframe also fatigue - but (barring damage or abuse) much more slowly than the pressure hull (if present).  That's why the limiting factor for a pressurised aircraft's lifespan is the pressure hull's fatigue life, rather than that of (say) the wing spars, fuselage keel beam, or stabiliser attachments.

Not.  The entire airframe will be designed to its target fatigue life whether the structure's getting bent or stretched.  Any more meat than the minimum required plus scatter factor is ballast.  The long term fatigue rig does the load cycles for everything; the pressure hull, bends the wings and tail, loads the engine mounts and landing gear trunnions, all for the same target life.   Often things like gear trunnions or wing planks start to crack long before the pressure hull is shot.

Who says the joint is rigid anyway? If the joint has natural flex in it, there will be no fatigue (in theory).

The "natural flex" will be aluminum being elastically deformed, like a spring, a little or a lot, so fatigue is always a potential factor.

How and why isn’t this a problem in practice?

The answer is in the question: in practice. There will always be stresses in the construction, including bending stresses. The double bubble just has much lower internal stress than a comparable oval shape
A fuselage cross-section under pressure will want to acquire a circular shape - if it starts out in a circular shape to begin with, the pressure applies pure tension on the cross shape skin. What you drew in the first image is not what happens, a circular shape does not transform into an oval one due to pressure differential.
The floor itself is only under expansional stretch from the skin bubbles, provided of course that pressure above and underneath the floor is equal.

The design issue mentioned in the question is about the stresses in the floor/fuselage intersection. If we model these as hinges, which by definition cannot absorb bending loads, we get the double bubble Homer Simpson-like pic above. This is the ideal design minimising all stresses. Minimising, not zeroing out.
Point is: nowhere is there a large additional bending moment introduced, like when an oval cross-section is under pressure.

...and if, like I stated in the question, the floor is less stretchy than the pressure hull, and, thus, expands less than the hull does when the aircraft is pressurised, or vice versa?

The hull will still expand into a circular shape, picture the connection between floor and skin acting like a hinge.

Which forces the connection to bend, in order to allow the angles between the three legs of the connection to change without the legs breaking off.

Yes - to a manageable extent though. These are all gross modo considerations, the detailed construction design will always have to accommodate for stresses. They just look to minimise them that's all. The beams underneath the floor experience bending stresses as well, and they're designed to accommodate them. Meanwhile, the designers spent a very considerable amount of time checking out how to minimise the bending stresses.


text:
What is the purpose of the Dash 8’s “TOUCHED RUNWAY” warning light?
The DHC-8-400 (and possibly other DHC-8 variants) has (or, at least, had in late 2007, according to Appendix G of this AAR) a warning light labelled “TOUCHED RUNWAY”.
The extract of the DHC-8-400’s QRH quoted in Appendix G of the aforementioned AAR mentions this warning light at the end of the landing-gear abnormal-procedures section, and says what to do if the light illuminates, but not what the light actually indicates:

”TOUCHED RUNWAY”
(Warning Light)

Do Not Start APU
Advice [sic] ATC for Runway Inspection

Note: Continued flight operations require maintenance approval.

The landing gear of a DHC-8-400 (or, indeed, of any aircraft so equipped) routinely touches the runway, and the ramp, and the taxiways, and the hangar floor, all the time during routine operations - indeed, touching the ground (including, but not limited to, the runway) is literally the primary, and very nearly the entire, purpose of landing gear!  So what does the “TOUCHED RUNWAY” warning light indicate that is severe enough to warrant possibly grounding the airplane?

The light is a tail-strike indicator.

Touched Runway system
The  Touched  Runway  system  provides  a  warning  light  in  the  flight  compartment.  It  illuminates when the aft fuselage makes hard contact with the runway on take off or landing. The system consists of a frangible1  switch located at the bottom of the aft fuselage, activating the warning light. The system is powered from the 28 Volt DC right main bus. Once activated, the frangible switch can not be reset but must be replaced by maintenance personnel. Source: Smart Cockpit.

1 frangible
/ˈfran(d)ʒɪb(ə)l/,
adjective

able to be broken into fragments; brittle or fragile.
"the frangible skull of an infant"
denoting ammunition designed to disintegrate into very small particles on impact.
    "frangible bullets"

Figure 1. Source: ATSB.au.

Figure 2. Source: ATSB.au.

Links:

Tail strikes during landing involving Bombardier DHC 8 402, VH-QOT and VH-QOS.
AAIB Bulletin: 10/2017 G-ECOJ EW/G2017/01/03.

What you're saying is that the "touched runway" light has nothing to do with the gear. It's a notification that there was a tail strike.

Correct. The switch is on the underside of the tail. See the update.

The B777 has an EICAS warning message "TAIL STRIKE" that is displayed when a tail strike is detected.  Seems like better wording than "touched runway".

There are a couple of other very obvious indicators of a tail strike in that photo :-D None of which are visible from the cockpit, of course, hence the sensor.

Now that's a hard tail strike.  Most of the time it's a little bump that scrapes off a bit of the bumper, if you're using proper landing technique and get it wrong a bit.   You wouldn't need a light to tell you about the one in the pic.

Judging by the amount of what looks like multiple strikes on that plane I think it would be worth carrying frangible switches in the hold.

@Transistor: Maybe it's a rental plane, just like renting a car to teach kids to drive a stick.

@Transistor as well as a few spare tail panels.

To supplement Transistor's answer:
The reason for the light is that tailstrikes are very easy to do on the 400 due to the stretch of the rear fuse with not enough landing gear lengthening to compensate, so they are more limited to the deck angle when landing and taking off than most a/c.  This isn't a problem on the other shorter -8s and is mostly unique to the 400.
When you land or take off a 400 you pitch to a certain attitude and avoid pitching any higher (I forget the number - 10 or 15 degrees or something like that) and on landing you control the touchdown by feathering off the power while holding the pitch attitude constant. You don't just go to idle and hold it off hold it off hold it off, pitching ever higher until the mains touch.
It's a bigger problem when landing at one of the lower landing flap settings (-8s don't need to be full flap only for landing and the lower settings are more efficient) which puts the deck angle higher in the first place.  Landing at full flap it's not so bad.  I know one acquaintance that has bumped the tail on the 400.  Not that big a deal.


text:
Why is the 737’s aileron/spoiler authority reduced at low flap settings?
According to the NTSB report on the crash of United Airlines Flight 585, the ability of the 737’s lateral controls (ailerons and flight spoilers) to counteract the rolling forces produced by (say) a rudder hardover is considerably worse at lower flap settings, such that, in some cases, a hardover occurring with the airplane’s flaps fully extended might produce only minor control difficulties, whereas the same malfunction occurring with the flaps at (say) the ten-degree setting would result in a loss of directional control (pages 60-61 of the report; pages 78-79 of the PDF file of the report):

...Flap position and airspeed are important when determining controllability during a rudder hardover condition. With the rudder at about 25 degrees airplane nose right (ANR), the following conditions would exist at 150 to 160 knots calibrated airspeed (KCAS). Bank angles are noted as left or right wing down (LWD, RWD) and provide constant heading trim solution (no turns), except for the last case.
Rudder Angle …… Flaps …… Side Slip Angle …… Wheel Angle …… Bank Angle
25 ANR …………… 40 ……… 14 ANR ……………… 35 LWD ……… 18 LWD
25 ANR …………… 30 ……… 15 ANR ……………… 44 LWD ……… 17 LWD
25 ANR …………… 25 ……… 15 ANR ……………… 68 LWD ……… 16 LWD
23 ANR* ………… 15 ……… 17 ANR ……………… 107 LWD ……… 23 LWD
21 ANR* ………… 10 ……… 16 ANR ……………… 107 LWD ……… 19 LWD
25 ANR** ………… 10 ……… 13 ANR ……………… 107 LWD ……… 40 RWD
* Less than full rudder allowed to maintain directional control.
** Loss of directional control.
At 10 and 15 degrees of flap setting, heading cannot be maintained with full rudder deflection. If full right rudder is achieved with a 10-degree flap setting, for example, heading control is lost and, according to Boeing, a steady 40-degree right-wing-down trim solution is attained that results in turning flight to the right even with full left wheel deflection...

Why, for a given airspeed, do the 737’s lateral controls become increasingly ineffective as the flaps are retracted?

Okay, turns out this wasn't so different from the crossover-airspeed question after all!
To quote @PeterKämpf's answer to that question:

how can [an airplane’s ailerons and spoilers] lateral control authority surpass that of the rudder past a certain speed?

This depends on the wing's lift coefficient. At a higher lift coefficient the lower aileron can not add the same amount of lift that it could at a lower lift coefficient. While the raised aileron on the opposite side will still reduce lift locally, the lowered aileron becomes less effective in raising lift as the lift coefficient increases. A higher lift coefficient causes a higher suction peak near the leading edge and puts more stress on the boundary layer, and adding more of the same will become harder as the wing approaches stall conditions.
Another factor is adverse yaw which increases with lift coefficient. This adverse yaw will add to the yawing moment of the hard-over rudder and will increase sideslip, which in turn will produce more rolling moment from dihedral against the aileron effect. As adverse yaw dies down with lower lift coefficient, so does sideslip angle and the ailerons gain control power.

Which also explains why the 737's crossover airspeed is lower with flaps down; when the flaps are up, the aircraft has to fly at a higher angle of attack in order to generate the same amount of lift at the same speed.  This increases the lift coefficient over the entire wing, including the outboard portion where the ailerons are mounted, which makes the downwards-deflected aileron, to use Peter Kämpf's terminology, "less effective in raising lift", and increases the adverse yaw generated by the deflected ailerons.  All of which decreases the roll authority of the ailerons, and, since the spoilers' roll authority can't increase to compensate, decreases the roll authority of the aircraft's lateral controls as a whole.
With the flaps down, on the other hand, the lift coefficient of the inboard wing increases, which means that the rest of the wing doesn't have to generate as much lift, which means that the aircraft can fly at a lower angle of attack for a given speed, which decreases the lift coefficient of the outer, flapless, aileron-containing section of the wing, which increases the effectiveness of the lowered aileron and decreases the adverse yaw generated by the ailerons, which increases the aircraft's roll authority.

Which boils down to "should the rudder be stronger than the ailerons".  I would think the trim shouldn't be, manual control, yes.  No way the rudder should "hard over" by trim only to the point the plane is unflyable at any speed.  If the plane is "fly by wire", something must be done (by design) to be able to maintain control of the plane.

Noted that directional control is maintained at higher flap settings due to unflapped aileron portion of wing at lower AoA.


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Was the Boeing 747 designed to be supersonic?
I heard a rumour that the Boeing 747 was originally designed for supersonic cruising.
Is this correct? Are the airframe and wings able to cross the sound barrier without damage?

It was designed for high sub-sonic cruise, not supersonic flight. There may be parts of the airplane that do go supersonic due to the airflow but the entire airplane does not.

You may be confusing it with the Boeing SST which was designed at the same time and was the companies other big bet for the future of aircraft. Boeing felt that the 747 would be quickly replaced with the SST when it was complete, that however, never happened.

To expand on what @Dave said, the Boeing 747 was intended to be a stopgap until they could produce an SST. Boeing didn't expect to produce more than 400 747s and expected those to be converted into freighters, which is the reason for the hump; the freight was loaded through the nose. As Dave notes, this is probably what you were thinking of.

No. The Boeing 747 was not designed for supersonic flight, though during testing, it was pushed quite close to the sonic speed:

Tom Cole, a spokesman at Boeing Commercial Airplane Co., said original flight tests of 747s conducted in 1969 and 1970 took 747-100 models to speeds of Mach 0.99.

However, the aircraft is not built for sustained supersonic speeds:

... Boeing and the FAA said the 747 is not built for sustained flight at the speed of sound. Its engines aren't powerful enough nor is it designed to deal with a destabilizing shock wave that develops around the speed of sound.

However, it is just possible that the airframe and wings can cope up with the supersonic flight, at least for a little while, though any sustained flight is not possible. There have been cases where the 747 has gone quite near the speed of sound and lived to tell the tale.
China Airlines Flight 006 went into a dive and went near supersonic speeds before the captain could recover it. As a result, large portions of the horizontal stabilizers were ripped off, and the wing was bent permanently upwards.

Damage to China Airlines Flight 006 empennage. By NTSB - China Airlines Boeing 747-SP Accident Report by NTSB, http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/ChinaAir/AAR8603.html, Public Domain, Link
In 1991, a Boeing 747 of Evergreen airlines entered a steep right wing bank and approached sonic speed before recovery was completed. NTSB notes:

... the airplane was in a steep right-wing-down bank. The flight  lost  approximately 10,000 feet of altitude, and the airplane approached supersonic speeds (0.98 Mach) before the recovery could  be  completed.

Later, the Israelis took it close to sonic speed. In these cases, parts of the aircraft certainly experienced local supersonic flow; however, the airframe and wings are not designed for supersonic flow nor are the engines powerful enough. In one case, the aircraft suffered extensive damage (though it was repaired and returned to service).

@ymb1 Thanks. Didn't notice it :)

While the China Airlines flight does seem to be incriminating, recovering from the dive may have done more damage than crossing the sound barrier. Hard to draw many conclusions from what is presented here.

Apart from the issues pertaining to the engines and heat causes by drag, why can't the frame handled sustained flight? I thought the only issue would be the momentary stress causes by the sonic boom. Once you're past that, what's to stop you?

@Alexander, the 'sonic boom' is a phenomenon observed only on the ground, when a moving shockwave passes through you. For the aircraft, this is a sustained experience. That said, even a piece of rock can fly supersonic if it has enough energy :)

@AlexanderMomchliov " It is a common misconception that only one boom is generated during the subsonic to supersonic transition; rather, the boom is continuous along the boom carpet for the entire supersonic flight." From Wikipedia

@sanchises Oh that's cool, makes total sense. Then that leads the question: How does >mach 1 cause stress on the air frame? AFAIK going just under mach 1 builds a high pressure zone immediately in front of the plane. Overcoming it would be hard from the force it applies back on the plane. But once you do, what happens?

"In these cases, parts of the aircraft certainly experienced local supersonic flow" - @aeroalias, all modern jetliners have cruising speeds in the low transonic range, and, therefore, experience supersonic flow over parts of their wings during normal operation.  That's why they use supercritical airfoils and are shaped to follow Whitcomb's area rule (hence the spectacular anti-shock bodies on some older designs).

The Boeing 747 is NOT designed for supersonic flight... However, it was designed as an interim aircraft while the SST(SuperSonic Transport) was being developed.
As you probably know, the SST never panned out, and the 747 is now one of the most influential aircraft in the world.  As a book I read put it, the 747 was likened to a Mack truck of aircraft; it wasn't glamorous, and the SST was supposed to steal the show.
Some good reading on the 747 if you want more would be
747: Creating the world's first jumbo jet, by Joe Sutter
Wide-Body: The triumph of the 747, by Clive Irving
It really is a fascinating story.  It was developed in a very short time, and the amazing part is that they got it right.
Anyhow, maybe you heard that it was supposed to be an interim aircraft and got it confused.
Hope this helped!

That's why the 747 has an upper deck - so it could easily be converted to a cargo plane when the SSTs came along.

Absolutely in no way was the 747 intended or designed as a supersonic aircraft; for many technical reasons having to do with structures and aerodynamic/fluid mechanics, a 747, if pushed to the sound barrier breaking speed in the vicinity of 674 MPH, plus or minus with respect to air density in which the big airliner is flying, or beyond this speed, then structural air drag stresses would rip the wings from the fuselage. In comparison, go look at old photos of the Concorde and note the triangular, or delta wing configuration of this aircrafts wings and note that the wings do not flare out in a wide wingspan is in the 747. As well, the running length of attachment of the Concorde delta wings to fuselage is much greater than that longitudinal length of attachment-to-fuselage in the 747. There were of many other material and design differences between these two airliners to allow the Concorde to fly at supersonic speeds, and conversely, disallow these faster than the speed of sound abilities in the 747.

Could "rip the wings from the fuselage"?  Of the various bad things that can happen when overspeeding a large jet, that seems very low on the list of what's most likely. The 747's have an MMO of M 0.92, which has a safety margin built in. This link reports that there are cases where one has gone inadvertently supersonic - apparently not losing its wings in the process.


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Do flight schools typically have dress codes or expectations?
I attend a local flight school for lessons once or twice a month. I normally wear a hoody and some tracksuit bottoms as they are comfortable for me and I like to be comfortable whilst doing something that I need to apply focus to (I dress similarly for my job as a developer).
Most people that are there for lessons are dressed very smartly, usually in a shirt and tie. I presume this is because they want to impress people, as they are attending the airport they hope to work at in the future (it's a big airport). I am just doing this for a hobby, but the fellow students often look me up and down, usually looking confused and/or disapproving.
Do flight schools typically or traditionally have more formal dress codes or expectations than this? Are there substantive reasons for why certain clothing choices would be required or expected?

Just don't actually be wearing a hoodie with hood pulled up in an aviation situation please!  Nothing says "I don't care about situational awareness and am not really interested in what is going on around me" louder than than, unless extreme cold demands it.

Perhaps it would also be wise to avoid synthetic clothing that is flammable or has a low melting point, as that would significantly add to your task load in the event of an in-flight fire.

I know that when JAR was replaced by EASA in the UK, flight schools had to have a dress code, though I think the specifics were left to the ATO   https://forums.flyer.co.uk/viewtopic.php?t=80302

I really don't see why this question has a close vote on it (allegedly, it's "opinion-based"). Whatever your opinion of dress codes, whether flight schools typically have dress codes or expectations is not a matter of opinion: either they do or they don't.

If you are learning to fly as a hobby and are paying the school your painfully earned money, you can dress however the hell you want, within reason.  Formal attire in that circumstance is ridiculous.  If you are in a professional school in some airline related program, or a government subsidized school with a formal dress code, that's another story, and you follow the rules as they are.
Anyway, the problem with hoodies and track pants is really loose clothing and hoods can be a problem in a cramped cockpit and especially if trying to get out in a hurry on the ground.
A cotton or wool shirt without a hood and good pockets, and cotton or wool cargo pants (cargo pant pockets are really handy) is my favourite dress for flying.  Avoid "plastic" synthetic fabrics.  Having fibres made from plastic, your clothes will melt into your skin if things get "hot".
Same with shoes.  Wear shoes that will let you walk at least some distance on a rough surface.  You may find yourself walking a mile over rough terrain after a forced landing.
Dress for comfort and contingencies, without getting carried away (like buying a nomex flight suit) in other words.

If there's a dress code (whether formally specified, or just one of expectations) in any situation, it's always a good idea to be aware of it, and usually a good idea to abide by it (unless you're confident that your remarkable deviation from the norm will come to change the dress code itself, but that's probably easier to achieve at a high-society event than at a flying school). Whether a dress code is ridiculous (as indeed some are) is a different question, but a dress code in a flying school is probably there for a reason.

"If you are learning to fly as a hobby and are paying the school your painfully earned money, you can dress however the hell you want" I disagree with the sentiment here. It smacks of "the customer is always right" entitlement. You are not their king just because you paid them your "painfully earned money". You and the instructors have agreed to exchange your money for their knowledge, experience, and pedagogy, which are also valuable and may also have been "painfully earned". You are both contributing something to this arrangement, so they are allowed to set expectations too if they wish.

The customer is always right because he's allowed to take his money elsewhere.  He's going to a small flight school.  No flight school like that is going to impose a "dress code" beyond certain safety related common sense practicalities, and drive off customers.  The idea of a private flight school, competing for business with other flight schools, imposing some silly dress code like shirt and tie to take recreational flying lessons is absurd.

The idea that the customer is always right when the customer is a student pilot is horrendous. Students are not and should not be customers. That is a very dangerous relationship, bad enough in an academic subject, potentially lethal in aviation. The student is there to learn to fly safely, not to be satisfied with their purchase as if they were buying a dinner. The duty of the educators is to provide the student with a good education, not to satisfy their wants. The student's free to go elsewhere if they dislike the contract, but let's not pretend it makes them "right" (or wrong).

The problem is you are interpreting that phrase literally.  The phrase refers to attitudes to customer service and they way the customer is handled and has nothing to do with being technically correct or incorrect about something.  A school competing for students that can cross the street to another one would be crazy to impose a dress code with no useful purpose other than the school owner's sense of aesthetics.

flight schools here laugh at students showing up in pilot uniforms, yet some people still think that's the proper way to dress even as a private pilot...

"Avoid "plastic" synthetic fabrics." I think that's obsolete thinking, I have some FR-rated clothing that's 100% polyester, as it's inherently flame resistant (doesn't rely on chemical treatment). If you want to be safe, low-level FR-rated light/casual work clothing and shoes are readily available, due to increasing workplace requirements.

If you get synthetic clothing treated with flame retardant, that's great. But you have to seek it out.  But for regular pants, the synthetics are going to be more formal wear anyway, like dress slacks. Hard to go wrong with cotton jeans or dockers, and a cotton T shirt.

Expectations
Learning to fly is not cheap, and you are likely (depending where you are studying of course) in a minority, if you are there as a hobbyist, because I would expect that most of the other students have a professional future as a pilot in their minds. For them, it is a serious business in all respects.
Professional pilots, in civil aviation or the military, will generally wear a uniform. The uniform, like much of the profession, is derived from naval traditions, where rigour and discipline are valued in appearance as much as in conduct and exercise of duties.
Dressing smartly doesn't mean someone is a better pilot, and dressing casually doesn't mean the opposite. However, attention to dress indicates attention to detail.
In aviation, precision and formality in thought, actions, speech and behaviour are key to safety and correct operation (we wouldn't be too happy to hear an airline pilot announce "Uh, yeah, we'll be, you know, heading in like an easterly direction an' that.")
If crisp formality is key to good piloting (and I think it is) then it seems reasonable to expect to hear it, feel it and see it all around you in flight school - including in the way fellow students dress.
Practical matters
Then there are more practical considerations. If everyone is dressed similarly, in something approaching a uniform or at least a standard, then it becomes easier to see when something is wrong or out of place (for example, if someone is unwell).
When actually flying or working around machinery, some attire is certainly suitable or unsuitable. Flappy garments that obscure instruments or catch controls or switches as you move are obviously dangerous. Shoes need to be appropriate for the job, and so on.
Depending on the kind of plane, the cockpit can also be a potentially hazardous environment, in which smoke, fumes, fire, fluid leaks and so on can occur.
A shirt and tie don't offer you better protection than a tracksuit from those (unless you like to wear, say, a nylon tracksuit), but wearing otherwise appropriate attire is a signal that you are generally aware of the environment and its demands and expectations.
School requirements
It could also be that your flying school formally sets out an expection or a requirement ("For classroom-based instruction, we expect [or even, require] male students to wear shirt and tie, etc etc.") and that somehow you have missed it - in which case perhaps your fellow students are aghast at your daring flouting of the rules.
It never hurts to ask: is there a dress code?

Besides, leaving dress codes to the side for a moment, one doesn't necessarily need to "dress up" to fly (or to learn to fly), but dressing appropriately is likely a good idea. If I was getting on a plane and someone who's going in the cockpit is wearing a dressing gown, I'd get off it in a hurry, regardless of whether it's the little Ikarus or Cessna at the local club or a commercially operated jetliner.

I have to disagree about people learning to fly for their own enjoyment being outnumbered by career-oriented people.  In my experience, and especially at smaller places, it's just the opposite.  FWIW, I don't recall wearing anything but jeans and a t-shirt during any of my training.

@Agent_L ...or have their employer (assuming they're flying commercially; otherwise, the pilots themselves, if they like to wear a tie while flying) invest in tie clips?

@jwenting You make it sound very rigorous and disciplined. What we really mostly do is the same thing over again in the hope of getting a different result.

Flight schools that are oriented toward career pilots may require (or at least strongly encourage) those students to wear a uniform to help reinforce the sense of professionalism, to prepare students for what they will face in the workplace, and to present a more professional look to attract like-minded students and impress their future employers. It is especially common if they are affiliated with an airline cadet program.
Hobby students will exist at (nearly?) all flight schools, though what fraction they represent will vary wildly. At some places, you might be the only one. At others, it may be everyone. I've never heard of a requirement for a uniform for hobby students, even at schools where they're uncommon.
However, as others have noted, your attire should still be appropriate for flying. For instance, a hoodie's large front pocket could get caught on the yoke/stick, causing a loss of control, and nylon track pants could catch fire. Jeans or shorts, golf-shirt or t-shirt, and a normal jacket are safer choices and are commonly seen on the ramp and in FBOs.

The FAA has an opinion on this although there are no hard regulations (so far as I know)

Speaking of clothing, this is one area often overlooked when it comes
  to surviving an aircraft accident. As clothing is your primary shelter
  in a survival situation, plan your attire accordingly for all areas
  and weather conditions along your route of flight. Dressing in layers
  is always a good idea. That way you can adjust as conditions change.
  Consider cotton or wool outer garments rather than synthetics,
  trousers rather than shorts or skirts, and closed toe shoes rather
  than sandals.

The two flight schools I trained at did not really care what students wore so long as it was in line with the above statement. Instructors were required to be in a company shirt of sorts and were usually well dressed. Slacks were not uncommon in the colder months but shorts were acceptable in the summer because the flight school owners understood the reality of a PA-28 interior on a hot July day...


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Approach plate does not require DME, but DME used in the published missed?
This approach plate doesn't say "DME REQUIRED", but the published missed approach uses DME.

Can I still fly it without DME equipment?
If this approach didn't say "RADAR REQUIRED", could you fly it without being in contact with ATC, and how would you manage the DME in the missed?

DME can be used in the published missed, but does not have to be. There are other ways to identify the missed approach fix.
The missed approach instructions are:

MISSED APPROACH: Climb to 1500 then climbing right turn to 3500 on ATL VORTAC R-005 to TROYS INT/ATL 15 DME and hold.

The missed approach procedure calls for a hold at the TROYS intersection. This can be identified by one of three means:

The intersection of the ATL R-005 and RMG R-115,
By 15 DME on the ATL R-005, or
By RNAV from a database.

These multiple methods of identification mean that DME is not required. If an RNAV or dual VOR receivers are used, DME is not needed.


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What is the distance at which my VHF communication radio can reach ATC?
I just installed 2 new TKM MX11 radios and a new Garmin 340 audio panel. I also put in new coax cable and 2 new antennas in my 1964 Skylane.  What kind of distance from ATC should I expect at 4 or 5 thousand feet?

This is a difficult question to answer, best thing to do is pick some stations that are certain distances away and go up and see if you can tune them in. Just remember, listening and transmitting that far are two different things, ground stations have a lot more power than your aircraft.

Why the downvotes on a first question?  Ridiculous.

Hello Joel Segal, welcome to aviation.stackexchange.com!

No idea why the downvotes, I thought it was quite a reasonable question, regardless of being Joel's first or not.

Possible duplicate of How do you calculate the maximum distance to communicate with tower control?

@fooot: The other question is about the maximum distance, and the answers indeed give the LoS formula. However communication is limited by both transmission power and receiver sensitivity, aerial gain, and sometimes by obstruction in Fresnel areas, etc. In my opinion, this question deserves a more complete answer than just calculating the maximum LoS.

@Simon I've no opinion on the quality of the question, so I didn't vote either way.  But up or downvotes should be based on the quality of the question, not on whether or not the user is new.

The real answer is a frustratingly tautological "the distance at which you can get an ATC response." Transmission varies massively depending on atmospheric conditions, terrain, altitude, the specific freq the equipment is optimized for, angle and orientation of transmit and receive antenna at the moment of broadcast, shielding and interference from other systems, background noise from solar flare activity, etc. Heck even the batch of transistors used by Garmin the day they made your radio.

The TKM MX11 has an 8 watt transmitter which makes it a Class 4 transmitter as defined in RTCA DO-186B MOPS for Airborne Radios Communications Equipment. That specifies a maximum range of 100 miles.
In an area of relatively level terrain, you should see a range of about that 100 miles if you're at 5000 AGL or above. Below that, range will be reduced due to the curvature of the earth and multipath interference. At 2000 feet above level terrain, 40 to 50 mile range is typical based on an analysis I did about 20 years ago for a former employer.
One factor you can't control is the height of the ATC antenna.  The higher it is, the greater the range. Even a 50 foot difference in the height can affect the range by several miles.

Do you know if the 100 miles are specified as nautical or statute miles? Anecdotally, I have received radio transmissions near ground level from aircraft at ~87 NM (~100 statute miles) distance that were at 5000 ft.

The wording in DO-186 is vague, so I looked up TSO C-169a which calls out the MOPS and defines the classes.  It states: Transmitter used in a 25 kHz channel separation environment and intended to operate with a range of 100 nautical miles. Similarly, when flying at 5000 ft along the Gulf coast I've been able to communicate with UNICOM from 100 NM, but I've not been able to at 120 NM except where the ground station antenna was significantly higher than local ground level.

The range of radio communication traffic depends on a number of factors.
VHF radio communication is approximately line-of-sight; the radio waves to not follow the curvature of the earth very well. This means in general that if the antenna of the other side is below the horizon, or behind a mountain you will have no reception.
A second aspect is the power of the radio transmission from transmitter to receiver. This depends on the power of the radio, losses in the cables, transmission antenna gain in the direction of transmission, attenuation in the transmission medium, free space path loss, receiver antenna gain in the direction of reception, losses in the cables and sensitivity of the receiver.
Also in the above, the noise levels need to be taken into account; AM transmissions are sensitive to radio noise.

The line of sight  distance can be calculated as:
$\sqrt{2Rh_{ac}+h_{ac}^2} + \sqrt{2Rh_{atc}+h_{atc}^2}  $
Where $R$ is the radius of the earth, $h$ the height of the antenna, subscript $ac$ the aircraft and subscript $atc$ the air traffic control antenna.
For a aircraft height of 3500 ft and an ATC antenna at 50 ft the line-of-sight distance is approximately 81.5 nautical miles.

With a rated output power of 8W for the MX11, an assumed cable loss of 2dB, estimated transmission antenna gain of 2 dBi (half dipole with some obstructions around it), receiver antenna gain of 2.14 dBi and a ICAO recommended receiver sensitivity of 20 microvolts per meter for ATC receivers, the transmission range is approximately 375 NM.

The transmission will be limited by the line-of-sight distance about 81 NM.


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I have really intense ear pain during descent in commercial flights. Would it be better or worse if I was flying a small non-pressurized plane?
Usually — but not always — after descending in a commercial jet airliner, my ears really hurt for several days, accompanied by minor hearing loss.
Nonetheless, I am still in love with aviation. I am thinking of starting my lessons to become an ultralight (UL) pilot. Could flying a non-pressurized plane actually make these problems worse?

False. UL airplanes usually will not climb very high(to avoid controlled airspace or other reasons) and climb rates are not so impressive either. Cabin altitude is about 10000ft inside an airliner. You can fly UL at 1000ft most of the time and enjoy the scenery unless you live somewhere in the mountains. I am a glider tug pilot and usually I don't feel anything though I drop from 800m(2600ft) at a maximum rate plane can handle exceeding 2000ft/min. I sometimes can feel the pressure difference in a commercial airplane.
Also you don't have to wait till you feel the pain. Just hold your nose with the fingers, close the mouth and try to blow as soon as you start to feel anything. It will release an inside pressure. Do it many times during the altitude change. If you wait to long if will be much harder or impossible to do that.
I towed gliders while sick and having running nose, cold, ect. No problems at all.

The general practice that passengers need to do to avoid ear pain is called ear clearing. If you're not able to use one ear clearing method, try some other ones.

Commercial airliners are generally pressurized to a cabin altitude of around 8000 feet when at high altitudes. So it depends a bit on where you live. If you live near sea level, you're feeling around 8000 feet of pressure change. If you live somewhere higher like Denver, you're only feeling around 3000.
Small unpressurized planes, especially ultralights, will tend to fly at lower altitudes, usually within a few thousand feet from the ground. If you live at a lower elevation, this will be much less pressure change than in an airliner. If you live at a higher elevation, it would be more comparable.
That being said, it may only take a few hundred feet of elevation change for the pressure to noticeably change, and each person may react differently. If you're worried about making commitments to flight training before you know how you'll handle it, you can go on a discovery flight, even just in a small Cessna.

The last sentence of this answer really is the most important IMO. Beyond generalizations, it's hard to tell exactly how unpressurized light aircraft movement will affect the OP specifically, but a discovery flight is a relatively low-cost ($100-$150 or even $200 really isn't much if you're contemplating getting a pilot's license), low-commitment option to find out whether it's likely to work out in one's specific case. It also allows for the option of the proverbial bailing out at almost any time by simply telling the pilot "I can't handle this, let's head back to the airport".

It is not only the absolute pressure (aka cabin altitude, an altitude at which the standard ambient pressure is the same as in the cabin) that counts, but the rate of change as well.
As other have said, the cabin altitude in typical non-pressurised GA airplanes will often be more favourable than in airliners, by the virtue of flying lower. But the cabin climb rate, and especially the descent rate (which we tolerate worse), can be much higher.
The pressurisation systems are typically programmed such that the cabin altitude rates didn't exceed ±500 fpm in normal operation, regardless of what the aircraft is doing. That is, even if the aircraft descends at -2000 fpm, the cabin altitude will 'descend' much slower. In the end, it only needs to 'descend' from (typically) 8000 ft, rather than from the airliner's 30000 ft.
Conversely, in a non-pressurised aircraft you'll have the same rate of change as the aircraft's own climb/descent rate. Touring GA aircraft typically descend at about -500 fpm, which is gentle enough. Yet now it depends on the pilot's skill and the task at hand. A fast descent even from a low altitude may hurt.
I don't have particular problems with altitude changes, but the most painful experience I ever had was similar to what @Andrius referred to: a very fast descent (or rather dive!) after dropping parachutists from 800 m, so that we land before them. Not only my ears, my eyes felt hurt!
In other words, if you are touring and not flying over mountains (above ~8000 ft), and have a minimally experienced pilot (or learn to fly yourself), you may feel better than in an airliner. But many kinds of special airwork, or an emergency, may pose a greater problem.